The invention relates to pyrogenically prepared silicon dioxide compacted to give crusts, to a process for the production of the crusts from pyrogenically prepared silicon dioxide, and also to their use.
It is known that pyrogenically prepared silicon dioxide can be prepared by means of high temperature or flame hydrolysis from SiCl4 or from other volatile silicon compounds (Ullmann's Enzyklopädie der technischen Chemie [Ullmann's encyclopaedia of industrial chemistry], 4th Edition, Vol. 21, page 464 (1982)).
Features of pyrogenically prepared silicon dioxides are extremely fine particles, low bulk density, high specific surface area, very high purity, spherical particle shape and the absence of pores.
Compacting of pyrogenically prepared silicon dioxide without binder is difficult because pyrogenically prepared silicon dioxide is very dry and there are no capillary forces that can bring about binding of the particles.
Pyrogenically prepared silicon dioxide often has high surface charge which acts electrostatically to make agglomeration more difficult. Furthermore, pyrogenically prepared silicon dioxide has a maximum moisture content of 1%.
It is known that colloidal silicon dioxide, which derives from the exhaust gas from silicon smelting ovens, can be subject to bulk-density increase by passing the colloidal silicon dioxide almost horizontally through a pair of vertically arranged compression rollers (U.S. Pat. No. 5,160,470).
That reference makes no mention of pyrogenically prepared silicon dioxide.
It is known that fine-particle substances can be subject to bulk-density increase by means of two screw compactors arranged at right angles to one another (U.S. Pat. No. 3,664,385).
It is known that powders, such as silica, can be subject to bulk-density increase by means of vacuum rolls arranged in groups (U.S. Pat. No. 3,632,247).
It is known that powders can be subject to bulk-density increase by means of a screw compactor and that the air escaping during that process can be returned in order to reduce powder loss (U.S. Pat. No. 5,052,874).
For certain applications and transport methods it is known that pyrogenically prepared silicon dioxides can be subject to bulk-density increase or can be formed to give pellets.
It is known that pyrogenically prepared silicon dioxide can be subject to bulk-density increase by means of the apparatus according to U.S. Pat. No. 5,052,874 (U.S. Pat. No. 6,156,285).
It is known that pyrogenically prepared silicon dioxide can be subject to bulk-density increase by using a compactor screw which has decreasing pitch (U.S. Pat. No. 4,326,852).
It is known that pyrogenically prepared oxides, such as pyrogenically prepared silicon dioxide, can be subject to bulk-density increase by means of gas-permeable belts (U.S. Pat. No. 4,325,686). The density achieved by this known process is less than 100 g/l.
U.S. Pat. No. 4,325,686, column 3, discloses that pyrogenically prepared silicon dioxide (fumed silica) can be subject to bulk-density increase by means of the apparatus according to U.S. Pat. Nos. 3,838,785, 3,742,566, 3,762,851 and 3,860,682. The maximum tamped bulk density given by this process is 180 g/l.
The known processes do not give defined compactates.
It is known that pyrogenically prepared silicon dioxide can be subject to bulk-density increase by means of a pressure-belt filter (EP 0280851 B1). However, this process likewise does not give defined compactates.
It is known that metal oxides whose primary particle size is from 1 to 500 μm can be formed to give compactates and that these compactates can be separately welded into foils in vacuo. These vacuum panels thus manufactured to a given specification can be used for thermal insulation, for example in refrigerators (U.S. Pat. No. 6,132,837).
It is known that pyrogenically prepared silicon dioxide can be subject to bulk-density increase by means of a transport screw (EP 0010655 A1). Again, this process does not give defined compactates.
It is moreover known that pyrogenically prepared silicon dioxide can be moulded to give pellets, by dispersing the silicon dioxide in water and spray-drying the material. These pellets can be used as catalyst supports (DE 196 01 415).
It is known that precipitated silica whose moisture content cannot be below 1% can be subject to bulk-density increase by means of filter rolls (DE B 1807714). The disadvantage of this process is that it can achieve bulk-density increase only when a silica comprises water.